gnu: Add python2-rdflib.

* gnu/packages/rdf.scm (python2-rdflib): New variable.
  (python-rdflib): Add patch.
* gnu/packages/patches/python2-rdflib-drop-sparqlwrapper.patch: New file.
* gnu-system.am (dist_patch_DATA): Register patch.

gnu/packages/patches/python2-rdflib-drop-sparqlwrapper.patch

@@ -0,0 +1,16 @@
+Drop SPARQLWrapper from the required install inputs under Python 2, as it
+creates a circular dependency.
+
+diff -u rdflib-4.1.2.alt/setup.py rdflib-4.1.2/setup.py
+--- rdflib-4.1.2.alt/setup.py	2014-03-04 12:40:26.000000000 +0100
++++ rdflib-4.1.2/setup.py	2015-01-23 21:52:59.000000000 +0100
+@@ -52,7 +52,7 @@
+         kwargs['test_suite'] = "nose.collector"
+         kwargs['install_requires'] = [
+             'isodate',
+-            'pyparsing', 'SPARQLWrapper']
++            'pyparsing']
+ 
+         if sys.version_info[1]<7:  # Python 2.6
+             kwargs['install_requires'].append('ordereddict')
+